Slow aperture on an SMC Takumar 75mm f/4.5

Hi, I just noticed that my 75mm Tak takes a while to return the aperture blades to the fully-open position; they move swiftly from the f/22 position until they hit about f/8 or f/11, after that they take up to few seconds to retract fully. Does anyone have any pointers on what might be the issue/get me started on disassembly?

That is a most common issue with old lenses in general. Most likely oil got between the leaves and made them stick to each other. This should be visible, though maybe only from one side. But stickiness may also occur at the actuating stage of the leaves and thus would be invisible.
In any case the lens must be disassembled. If you are lucky complete assemblies can be taken off to get access to the diaphragm stage.

No oil on the blades as far as I can see; but I did feel some kind of resistance when actuating the stop-down pin. However, the problem magically disappeared after turning the aperture ring from f/22 to f/4.5 and back, and I can't get it to manifest again. Oh well. Maybe something loose in there.

You might "repair" a sticky diaphragm if the blades are not oily by actuating the automatic diaphragm countless times. But likely the fault will return. Sooner or later...

slow blades may be from oil on the blade operating ring which you can't see without taking the shutter apart.
The BOR is the ring the blades are installed in. they have a pivot and slot, the blades have two pins, on fits in each position
and when the ring rotates the pin in the slot will move the blades.

I've repaired several sticky aperture blades in lenses before. I'd definitely get them repaired if the problem comes back because I've seen a few cases where the blades bind up and get bent trying to turn the ring. And once those blades are bent, they're pretty much useless. And replacement blades are often not easy to come by, and even harder to fabricate.

You can try to clean them yourself, but it's a fiddley job and not for the faint of heart. So it might be best to send it in to a pro. Though if you have good eyesight, steady hands, lots of patience, the right tools, and a clean place to disassemble a lens, it is possible to DIY.